1. Technical Field
The present invention relates to automatic repeat request (ARQ) methods for high-speed data communications in a mobile communication system and, more particularly, for use on the uplink during a soft handover.
2. Discussion of Related Art
In a standard method of checking transmitted data, the sender encodes an error detection field based on the contents of the message. The receiver recalculates the check field and compares it with that received. If they match, an xe2x80x9cACKxe2x80x9d (Acknowledgement) is transmitted to the sender. If they do not match, a xe2x80x9cNACKxe2x80x9d (Negative Acknowledgement) is returned, and the sender retransmits the message. Naturally, the sender at least temporarily stores the data it has sent until it is assured that the receiver has received it correctly. In the new third generation wireless system which employs code division multiple access, there will be high-speed data transfers using both circuit- and packet-switched data. For the noisy channels encountered in such systems, hybrid automatic repeat request schemes are contemplated for non-real time services. The above-described automatic repeat request (ARQ) idea is combined with forward error correction to arrive at hybrid ARQ (HARQ). Forward error correction (FEC) is a technique used by a receiver for correcting errors incurred in transmission without requiring retransmission. It typically involves a transmitter using a convolution algorithm to embed sufficient redundancy in the data block to allow the receiver to correct transmission errors.
There are different ways of implementing the automatic repeat request (ARQ) concept, but the selective-repeat continuous ARQ is probably going to be used in connection with FEC for third-generation wireless systems. There are several types of ARQ that can be used. For instance, Type I is defined as identifying an erroneous packet data unit and retransmitting an identical copy. The FEC coding rate is fixed, and there is no combining of earlier and later versions of the packet data unit. In Type II, the erroneous packet data unit that needs to be retransmitted is not discarded, but combined with some incremental redundancy that is provided by the transmitter for subsequent decoding. The retransmitted packets usually have higher coding rates than Type I and are combined at the receiver with the stored values. Type III is similar to Type II, except that every retransmitted packet is now self-decodable. In situations where the transmitted packet can be severely damaged due, for instance, to interference, it is better to have a scheme where retransmitted blocks are self-decodable.
In any event, in the third generation there is also a concept called soft handover, where a mobile station is connected at the same time to more than one base station, called xe2x80x9cNode Bxe2x80x9d in the third generation. Since third generation is a code-division multiple access system, it differs from earlier generation time division multiple access systems, where a handover was a short procedure, and the normal state of affairs was a non-handover situation. In CDMA systems, the situation is quite different, in that a mobile station can spend a lot of time in handover, called a soft handover (SHO) state. Each separate link from a base station is called a soft-handover branch and, from the point of view of the mobile station, there is not much difference between being connected to one Node B or several Node Bs. The mobile station is prepared to receive numerous multi-path components of the same signal using its specialized receiver. Since all base stations use the same frequency in a soft handover, the mobile station considers the signals as simply multi-path components (except for being coded with different spreading codes).
All of the base stations involved in an SHO are designated as being in an xe2x80x9cactive setxe2x80x9d. When the signal strength of a base station transmission exceeds an xe2x80x9caddition thresholdxe2x80x9d in the mobile station, the base station is added to the active set, and the mobile station enters into an SHO state, if not already there. A xe2x80x9cdrop thresholdxe2x80x9d prevents premature removal of base stations from the active set, i.e., it has a hysteresis effect because its value is always lower than the add threshold. A timer starts when the signal strength drops below the drop threshold, and the base station is removed from the active set if the timer expires. The network controls the values of these thresholds by communications with the mobile stations.
When in soft handover, with the mobile station communicating on multiple uplinks with multiple corresponding base stations, and when employing HARQ in such a communication, the base stations will be signaling to the mobile station on corresponding downlinks to inform it as to whether or not its transmissions have been correctly received or not. In such a case, one or more of the multiple base stations might have received the transmission from the mobile station correctly, while others may not. This can be a problem, since the contemplated high-speed services provided by third generation systems may be slowed down considerably if one of the base stations is able to hold up the successful transmission simply because it or more than one base station did not receive a particular transmission correctly, while one or more other base stations did receive a correct reception.
One way to address this problem, for instance in a case where there are two base stations in the active set, is to have the mobile station deem a transmission successful if either base station ACKs. See xe2x80x9cHarmonized 1xEV-DV Reverse Link Solution,xe2x80x9d C50-RL-20010523-xxx, May 23, 2001 by Lucent Technologies, Inc. Either one or all active set base stations may schedule a mobile station in soft handover. For the scenario with one base station doing the scheduling (timing of sending a packet) as well as assigning rate, duration and packet size, it is assumed the mobile was being served by that base station alone before it moved into handoff. The other base station does not schedule the user even though it is in the mobile""s active set. It however monitors the mobile""s reverse channels. The mobile responds to the scheduling by transmitting a data packet at the granted time using the appropriate power, rate and duration indicated by the scheduling base station. Both base stations decode the packet and send an ACK or NACK as the case may be. The mobile deems the transmission successful if either base station ACKs. In the next transmission, the mobile sends a message to flush out the buffer of any base station that may have been unsuccessful in decoding the previous transmission. For a case of both stations scheduling independently, the mobile may for instance use the scheduling received from the first base station and ignore the other base station""s schedule grant. Again, the mobile deems the transmission successful if for instance the first base station ACKs and the second NACKs. The next packet transmission of the mobile can be scheduled by either base station. The mobile sends a message to flush out the second base station""s buffer during this transmission.
A problem with this approach is that as individual base stations make scheduling decisions, then for example two base stations can assign the maximum rate to two mobiles simultaneously even when those mobiles are connected both to these two base stations. Then both mobiles can transmit at the cell border (in soft handover) at full rate and probably both the transmissions will fail. On the other hand, if one base station sets the values very low, then data rates could remain too low, especially if the active set is large.
An object of the present invention is to overcome the above-described problem by setting forth a procedure for packet retransmission handling in mobile communication systems in soft-handover.
According to a first aspect of the present invention, a mobile station has means that uses a method when operating in a packet-switched mode in a mobile communication system in which the mobile station is in communication over multiple radio links with corresponding base stations, that is characterized by determining, for a packet sent on uplinks of the multiple radio links to the corresponding base stations, whether an acknowledgement has been received on any downlink from any one of the base stations, and if so, proceeding to a new packet with a rate selected according to a rule from data rate information or signaling received from one or more of said base stations and, if not, determining if a negative acknowledgement has been received on a selected number of downlinks of the multiple radio links from the base stations and if so, proceeding with a retransmission of the packet.
In further accord with the first aspect of the invention, the mobile station and method are characterized by repeating the above-mentioned steps of determining, if a negative acknowledgement is not received from any base station or alternatively is received but not from the selected number of base stations, until acknowledgement is received, until negative acknowledgements are received from the selected number of base stations or until a timer started after sending the packet expires.
According to a second aspect of the invention, a base station has means that uses a method when operating in a packet-switched mode in a mobile communication system in which a mobile station is in communication over multiple radio links with corresponding base stations, wherein the base station checks for correct reception of incoming packets and sends an acknowledgement to the mobile station in case of correct reception and sends a negative acknowledgement otherwise, that is characterized by, prior to said check for correct reception, checking a block number of a newly-received packet, by determining if the block number is the same as a previously-received packet, by combining the newly-received packet with the previously-received packet if the block number is the same, and otherwise by storing the newly-received packet without combination.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.